Systems and methods for content-aware load balancing

ABSTRACT

Improved load balancing techniques are disclosed. For example, in one illustrative aspect of the invention, a method of satisfying requests in a system comprised of a plurality of servers comprises the following steps. At least one load balancer is provided for routing requests to the plurality of servers. At the at least one load balancer, a request sent from a client is obtained. At the at least one load balancer, the request is examined. Costs of satisfying the request by at least two of the plurality of servers are estimated. The estimation is based on at least one of a number and a cost of at least one remote access for satisfying the request. The request is routed to a server of the plurality of servers with a low estimated cost of satisfying the request.

FIELD OF THE INVENTION

The present invention generally relates to information systems and, moreparticularly, to techniques for content-aware load balancing in suchinformation systems.

BACKGROUND OF THE INVENTION

In general, an information system is a data processing system thatprovides some form of response to a user upon a user's request. TheInternet or World Wide Web (WWW or the “web”) is easily the mostubiquitous information system that exists today.

Scalable web sites associated with the Internet typically comprise oneor more load balancers for routing requests to a plurality of servers.The techniques used for load balancing the requests can have asignificant effect on performance of the overall system. If requests arerouted in a content-aware fashion, then the load balancer is aware ofthe contents of a request and can make more intelligent routingdecisions.

One of the drawbacks to content-aware routing compared withcontent-unaware routing is that content-aware routing usually incurssignificantly more overhead. Therefore, the benefits for performingcontent-aware routing must be significant enough to justify the higheroverhead.

Content-aware routing techniques have been proposed, for example, asdescribed in V. Pai et al., “Locality-Aware Request Distribution inCluster-Based Network Servers,” Proceedings of ASPLOS-VIII, October1998, the disclosure of which is incorporated by reference herein.However, the content-aware routing disclosed in the above-referencedwork is primarily intended for static requests and thus uses techniquesfor selecting servers which are not always well suited for dynamicrequests. Techniques described in C. S. Yang et al., “Efficient Supportfor Content-Based Routing in Web Server Clusters,” Proceedings of the2nd USENIX/IEEE Symposium on Internet Technologies and Systems (USITS'99), the disclosure of which is incorporated by reference herein,provide content-aware routing only for static content. Thus, existingwork in content-aware routing is not sufficient to handle the datapartitioning problems which occur in major deployments.

Accordingly, a need exists for techniques which overcome theabove-mentioned and other limitations associated with existingcontent-aware routing techniques.

SUMMARY OF THE INVENTION

The present invention provides improved load balancing techniques.

For example, in a first aspect of the invention, a method of satisfyingrequests in a system comprised of a plurality of servers comprises thefollowing steps. At least one load balancer is provided for routingrequests to the plurality of servers. At the at least one load balancer,a request sent from a client is obtained. At the at least one loadbalancer, the request is examined. Costs of satisfying the request by atleast two of the plurality of servers are estimated. The estimation isbased on at least one of a number and a cost of at least one remoteaccess for satisfying the request. The request is routed to a server ofthe plurality of servers with a low estimated cost of satisfying therequest.

The step of routing may be accomplished by classifying a request into apartition and routing the request to a server hosting the partition. Inthe step of providing at least one load balancer, the at least one loadbalancer may comprise at least one content-unaware load balancer routingrequests to a plurality of content-aware load balancers. Further, in thestep of providing at least one load balancer, at least one of theplurality of content-aware load balancers may reside on at least one ofthe plurality of servers.

The step of estimating costs may further comprise the steps of examiningat least one parameter included in the request and using informationabout how data are partitioned among the plurality of servers toestimate at least one of numbers and costs of remote accesses forsatisfying the request. The step of obtaining a request may furthercomprise obtaining a request that is sent using the Transmission ControlProtocol/Internet Protocol and the step of examining the request mayfurther comprise accepting a TCP connection. The step of estimatingcosts may comprise estimating at least one of a measure of resourceutilization and a desired service level for satisfying the request.Further, the step of estimating costs may vary with respect to at leastone of a number of servers and time. Still further, in the step ofobtaining, at the at least one load balancer, a request from a client,the request may comprise a request for dynamic data.

In a second aspect of the invention, a method of satisfying requests ina system comprised of a plurality of servers comprises the followingsteps. At least one load balancer is provided for routing requests tothe plurality of servers. At the at least one load balancer, a requestsent from a client is obtained. At the at least one load balancer, therequest is examined. Costs of satisfying the request by at least two ofthe plurality of servers are estimated. Information obtained in theestimating step is sent from the load balancer to a server of theplurality of servers along with the request. At the server, theinformation is used to satisfy the request.

In a third aspect of the invention, a system for satisfying requestsfrom at least one client comprises a plurality of servers, at least onecontent-aware load balancer for routing requests to the plurality ofservers, and at least one cost analyzer associated with the at least onecontent-aware load balancer for estimating costs of satisfying requestsby different servers based on at least one of numbers and costs ofremote accesses for satisfying requests.

In a fourth aspect of the invention, apparatus for satisfying requestsfrom at least one client in a system comprised of a plurality of serverscomprises a memory and at least one processor coupled to the memory andoperative to: (i) obtain a request from a client; (ii) examine contentof the request; and (iii) estimate costs of satisfying the request by atleast two of the plurality of servers based on estimating at least oneof a number and a cost of at least one remote access for satisfying therequest. The request may then be routed to a server of the plurality ofservers with a low estimated cost of satisfying the request.

In a fifth aspect of the invention, a method of satisfying requests fromat least one client in a system comprised of a plurality of serverscomprises the following steps. A request is obtained from a client.Content of the request is examined. Costs of satisfying the request byat least two of the plurality of servers are estimated. The estimationis based at least one of a number and a cost of at least one remoteaccess for satisfying the request. The request may then be routed to aserver of the plurality of servers with a low estimated cost ofsatisfying the request.

In a sixth aspect of the invention, an article of manufacture for use insatisfying requests from at least one client in a system comprised of aplurality of servers comprises a machine readable medium containing oneor more programs which when executed implement the steps of obtaining arequest from a client, examining content of the request, and estimatingcosts of satisfying the request by at least two of the plurality ofservers based on estimating at least one of a number and a cost of atleast one remote access for satisfying the request. The request may thenbe routed to a server of the plurality of servers with a low estimatedcost of satisfying the request.

In a seventh aspect of the invention, a method of providing a loadbalancing service comprises the step of a service provider providing aservice to a customer which comprises obtaining a request from a client,examining content of the request, and estimating costs of satisfying therequest by at least two of the plurality of servers based on estimatingat least one of a number and a cost of at least one remote access forsatisfying the request. The request may then be routed to a server ofthe plurality of servers with a low estimated cost of satisfying therequest.

These and other objects, features and advantages of the presentinvention will become apparent from the following detailed descriptionof illustrative embodiments thereof, which is to be read in connectionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a server system architecture accordingto an embodiment of the present invention;

FIG. 2 is a diagram illustrating a system for scaling a content-awareload balancer, according to an embodiment of the present invention;

FIG. 3 is a diagram illustrating a system in which content-aware loadbalancers are integrated with servers, according to an embodiment of thepresent invention;

FIG. 4 is a diagram illustrating a method for load balancing requests,according to an embodiment of the present invention;

FIG. 5 is a diagram illustrating a method for selecting a server basedon cost, according to an embodiment of the present invention; and

FIG. 6 is a diagram illustrating a computing system in accordance withwhich one or more components/steps of a load balancing system may beimplemented, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will be explained below in the context of anillustrative Internet or web-based client-server environment. However,it is to be understood that the present invention is not limited to suchInternet or web implementations. Rather, the invention is more generallyapplicable to any request-based environment in which it would bedesirable to provide enhanced load balancing performance.

Furthermore, content that is to be served in response to a request maybe referred to generally herein as an “object.” An “object” may take onmany forms and it is to be understood that the invention is not limitedto any particular form. For example, an object may be an electronicdocument such as one or more web pages. One skilled in the art could usethe invention in a variety of different electronic document formatsincluding, but not limited to, HTML (HyperText Markup Language)documents, XML (eXtensible Markup Language) documents, text documents inother formats, and binary documents. Also, the phrase “electronicdocument” may also be understood to comprise one or more of text data,binary data, one or more byte streams, etc. Thus, the invention is notlimited to any particular type of data object. Still further, it is tobe understood that the term “overhead” may include, but is not limitedto, computer CPU (central processing unit) cycles, network bandwidthconsumption, disk, I/O (input/output), etc.

Referring initially to FIG. 1, a server system architecture, accordingto an embodiment of the present invention is illustrated. As shown, oneor more clients (e.g., 102-1 . . . 102-M) send one or more requests to aserver system 103. It is to be appreciated that the clients (by way ofexample only, personal computers, personal digital assistants, cellularphones, etc.) may be coupled to server system 103 via the Internet orsome other wired and/or wireless communications network. The inventionis not limited to any particular communications network. Also, it is tobe understood that the term “request” is not limited only to requestsfor the return of data content from the server system. Depending on theparticular application, a request may have different purposes and/orseek different results. The invention is not limited to any particulartype of request.

Server system 103 includes a load balancer 104, a cost analyzer 106, anda plurality of servers 108-1 . . . 108-N. Cost analyzer 106 aids loadbalancer 104 in determining which one of the plurality of servers arequest should be routed to. The load balancer may be content aware. Inother words, load balancer 104 may have the ability to determine thecontents of a request in order to make more intelligent routingdecisions.

A content-aware load balancer can incur significantly more overhead forhandling requests than a content-unaware load balancer. For example, ifcommunication is taking place via Transmission Control Protocol/InternetProtocol (TCP/IP), then a content-aware load balancer would typicallyaccept a TCP connection in order to examine the contents of a request.This step incurs considerable overhead and would normally not berequired by a content-unaware load balancer.

Principles of the invention provide features for scaling upcontent-aware load balancers to handle high request rates. FIG. 2depicts one such feature.

Referring now to FIG. 2, a system is illustrated for scaling acontent-aware load balancer, according to an embodiment of the presentinvention. As shown, server system 202 includes a content-unaware loadbalancer 204 and a plurality of content-aware load balancers 206-1 . . .206-P.

Requests received from one or more clients (not shown) are initiallyrouted to content-unaware load balancer 204 which subsequently routesrequests to one or more of the plurality of content-aware load balancers206-1 . . . 206-P. The content-unaware load balancer has higherthroughput than any of the individual content-aware load balancers. Avariety of schemes may be used by the content-unaware load balancer forsending requests to the content-aware load balancers including, but notlimited to, round robin or methods considering the load on thecontent-aware load balancers.

Referring now to FIG. 3, a generalization of FIG. 2 is depicted in whichcontent-aware load balancers are integrated with (e.g., reside on) oneor more servers. That is, as shown, server system 302 includescontent-unaware load balancer 304 and a plurality of content-aware loadbalancers 306-1 . . . 306-R respectively integrated with a plurality ofservers 308-1 . . . 308-R.

Referring now to FIG. 4, a method 400 is illustrated for load balancingrequests, according to an embodiment of the present invention. Theserver system architecture of FIG. 1 will be referenced to illustratethe steps of method 400. However, it is to be appreciated that otherserver systems architecture (e.g., those shown in FIGS. 2 and 3, as wellas others not expressly shown) may be employed.

In step 402, a load balancer (e.g., 104 of FIG. 1) receives a request.In step 404, a cost analyzer (e.g., 106 of FIG. 1) examines the requestand identifies a server (e.g., 108-1 . . . 108-N) which will likelyincur a low cost for satisfying the request. In step 406, the request isrouted to a server identified in step 404.

There are a variety of ways in which step 404 may be implemented. FIG. 5depicts one illustrative method.

Referring now to FIG. 5, a method 500 is illustrated for selecting aserver based on cost, according to an embodiment of the presentinvention. In step 502, the request is examined. If the TCP/IP protocolis being used for communication, step 502 may involve accepting a TCPconnection. The request may contain one or more parameters.

In one embodiment, data are partitioned among the servers 108-1 . . .108-N asymmetrically. The proper server to handle a request depends onthe request. For example, suppose that data are partitioned based on aname parameter. If there are three servers, then one server could handlenames beginning with A through I, a second server could handle namesbeginning with J through R, and a third server could handle namesbeginning with S through Z.

When a request is received by the load balancer (e.g., step 402 of FIG.4), in the scenario described above, a parameter corresponding to therequest would contain the name. For example, if the name was “Smith,”then the request would be routed to the third sever. If the name was“Jones,” then the request would be routed to the second server.

In another scenario, the data are partitioned among the servers (108-1 .. . 108-N) in a more complicated fashion. A request routed to a servermay result in a number of different accesses to non-local data dependingon how the request is routed. By “non-local,” it is meant that the datasought are not stored at the server that initially receives the request,but rather is stored on a server or other network element remote fromthat server. For example, if the request is routed to the first server,this might result in three remote accesses. If the request is routed tothe second server, this might result in one remote access. If therequest is routed to the third server, this might result in eight remoteaccesses. These determinations would be made in step 504 based on thecontents of the request.

It is to be appreciated that one of the features of the invention, butnot the only feature, which distinguishes it from existing techniquessuch as, for example, the ASPLOS-VIII and USITS'99 papers mentionedabove is that the inventive techniques can be used for requests fordynamic data as well as for static data. By contrast, theabove-referenced papers describe content-aware routing techniques whichare only well-suited for static data. A request for static data is arequest for data, such as a file, which exists at the time that arequest is made. A request for dynamic data is one in which program codeis executed in order to satisfy the request. For example, ordering anitem at an e-commerce web site would typically be implemented as atleast one dynamic request. The order might result in several databaseaccesses, persistent state changes at the server, and an acknowledgementwhich is generated on-the-fly and sent back to the client in response tothe order.

Satisfying dynamic requests is typically much more complicated thansatisfying static requests. The overhead for dynamic requests is alsogenerally much higher. The inventive methods used for selecting serversare thus different from those which have been proposed for selectingservers for static content.

An application may be analyzed to determine how it can best bepartitioned to run on a set of servers. Partitions are often defined tohave little interactions with other partitions and thus the stateneeding to be shared is minimized. Logically, each partition may beassociated with a part of a computational task, which may include thecode and state to process a specific set of requests.

Partitioning can be done statically by analyzing the business logic ofthe application. These partitions can be further refined at run timebased on on-line workload statistics. Thus, the cost analyzer not onlyis preferably aware of the initial partitions but also may implement thelogic to repartition the application online.

The cost analyzer may take a global view of the application to determinehow to partition the application to minimize the cost of processing aparticular set of requests. The cost of processing requests generallyincludes central processing unit (CPU) overhead and also thecommunication overhead of synchronizing the underlying state that may beshared by several backend servers. Thus, the cost analyzer may alsodetermine how widely a piece of data should be replicated. Whilereplicating a piece of data widely may increase the overall capacity ofprocessing the requests associated with this piece of data, it may alsoincrease the cost of synchronizing the data across all of the replicas.Thus, it is desirable for the cost analyzer to balance consistencyrequirements, request rates, and synchronization patterns of the data.

Conversely, not-replicating data may also be considered by the costanalyzer. It may be more advantageous to route requests for particulardata to one or a few servers hosting said data in order to avoid datareplication costs. Further, the aggregate caching capabilities of a setof severs may be better utilized. For example, each of three serverscomprising a server set, each equally capable of servicing any onerequest, may be able to cache only one of three frequently used largeobjects at any one time due to size constraints. The cost analyzer maychoose to place large object A on server 1, large object B on server 2,and large object C on server 3. Subsequently, requests for each objectwould be routed to a server according to its cached location. In thisexample, each large object is able to be cached on at least one serverwhich results in overall improved performance. Thus, the cost analyzercan improve horizontal scalability of applications by making intelligentdecisions about requests for service and where to service them basedupon request content.

The underlying data held by servers may need to be migrated beforerequest routing can be changed. Migrating underlying data typicallycomes with a cost. Thus, the cost analyzer may also take the cost ofstate migration into account to optimize routing.

Since remote accesses are costly, it is desirable to minimize them. Instep 506, the system selects a server to route the request to based onthe costs determined in step 504. In the example from the previousparagraph, the second server would normally be the one selected since itrequires only one remote access. If the second server is highly loadedcompared to the first server, however, the first server might beselected in certain cases.

When the system determines costs of routing requests to differentservers in step 504, this determination does not have to be completelyaccurate. In several cases, an estimate will suffice. In performing thisdetermination, the system might execute code that determines parametersof the request, which servers need to be contacted to satisfy therequest, or other information that can help satisfy the request. Astraightforward approach would be to determine this information once bythe cost analyzer and a second time by the server to which the requestis routed. This approach incurs overhead due to the redundantcalculations. An optimization to alleviate these redundant calculationsis for the cost analyzer to store the relevant information in therequest and send the augmented request to a server. The server thenaccesses information stored in the augmented request by the costanalyzer to obtain relevant information for satisfying the request andavoiding redundant calculations. A compiler can perform programtransformation techniques on the program code for satisfying requests tofully or partially automate this optimization.

The cost analyzer may consider partition definition, partition-to-serverassignment, and desired optimization. Partition definition is theprocess of classifying requests into partitions. Partition-to-serverassignment is the process of deciding on which server a classifiedrequest is to be handled (in other words, on which server a partition isto be located). Desired optimization considers how to best distributepartitions amongst available servers. Individual partitions can be movedfrom one server to another on demand.

The cost analyzer can make dynamic determinations for each of thecontrol variables. For example, cost analyzer off-line analysis ofsystem utilization may recommend a different classification intopartitions scheme; or recommend a different allocation of partitions toservers. Further, cost analysis results may be different for varyingnumbers of servers in the server set, and may vary over time.

For varying numbers of servers, one partition-to-server allocationscheme may be optimal for two servers, another for three servers,another for four servers, and so forth. For example, say the partitionsare named {0, 1, 2, 3, . . . 9 }. For two servers, cost analysis maydetermine that optimal assignment is for partitions 0-6 to be assignedto server 1 and 7-9 to server 2. For three servers, the cost analyzermay recommend partitions 0-3 be assigned to server 1, partitions 4-7 beassigned to server 2 and partitions 8-9 be assigned to server 3.

For time variability, the desired cost optimization function may specifyone assignment of partitions to servers for the period 8AM to 5PM, thenanother assignment during the period 5PM to 8AM.

The cost analyzer need not optimize on “best” utilization of resourcesfrom the system's perspective. A simple service request load balancingtechnique to utilize a system is to round robin requests to each in aset of servers; or to choose the server with the least utilized CPU.Instead, the cost analyzer may classify requests and route them basedupon said classification results, even though this may not result in the“best” utilization from the system's perspective.

Cost analysis may consider quality of service requirements. For example,a “gold” customer may be directed to a “fast” speed partition, a“silver” customer may be directed to a “medium” speed partition, and a“bronze” customer may be directed to a “slow” speed partition. That is,the cost analyzer may not always seek to optimize from the system'sperspective, but rather relative to the request's importance.

It is to be further appreciated that the present invention alsocomprises techniques for providing load balancing services. By way ofexample, a content provider agrees (e.g., via a service level agreementor some informal agreement or arrangement) with a customer or client toprovide content. Then, based on terms of the service contract betweenthe content provider and the content customer, the content providerprovides content to the content customer in accordance with one or moreof the load balancing methodologies of the invention described herein.

Referring finally to FIG. 6, a computing system is illustrated inaccordance with which one or more components/steps of a load balancingsystem (e.g., components and methodologies described in the context ofFIGS. 1 through 5) may be implemented, according to an embodiment of thepresent invention. It is to be understood that the individualcomponents/steps may be implemented on one such computer system, or morepreferably, on more than one such computer system. In the case of animplementation on a distributed computing system, the individualcomputer systems and/or devices may be connected via a suitable network,e.g., the Internet or World Wide Web. However, the system may berealized via private or local networks. The invention is not limited toany particular network.

Thus, the computing system shown in FIG. 6 represents an illustrativecomputing system architecture for a load balancer, a server, a costanalyzer, and/or combinations thereof, within which one or more of thesteps of the load balancing techniques of the invention may be executed.

As shown, the computer system 600 may be implemented in accordance witha processor 602, a memory 604, I/O devices 606, and a network interface608, coupled via a computer bus 610 or alternate connection arrangement.

It is to be appreciated that the term “processor” as used herein isintended to include any processing device, such as, for example, onethat includes a CPU and/or other processing circuitry. It is also to beunderstood that the term “processor” may refer to more than oneprocessing device and that various elements associated with a processingdevice may be shared by other processing devices.

The term “memory” as used herein is intended to include memoryassociated with a processor or CPU, such as, for example, RAM, ROM, afixed memory device (e.g., hard drive), a removable memory device (e.g.,diskette), flash memory, etc.

In addition, the phrase “input/output devices” or “I/O devices” as usedherein is intended to include, for example, one or more input devices(e.g., keyboard, mouse, etc.) for entering data to the processing unit,and/or one or more output devices (e.g., speaker, display, etc.) forpresenting results associated with the processing unit.

Still further, the phrase “network interface” as used herein is intendedto include, for example, one or more transceivers to permit the computersystem to communicate with another computer system via an appropriatecommunications protocol.

Accordingly, software components including instructions or code forperforming the methodologies described herein may be stored in one ormore of the associated memory devices (e.g., ROM, fixed or removablememory) and, when ready to be utilized, loaded in part or in whole(e.g., into RAM) and executed by a CPU.

Although illustrative embodiments of the present invention have beendescribed herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various other changes and modifications may bemade by one skilled in the art without departing from the scope orspirit of the invention.

1. A method for satisfying requests in a system comprised of a pluralityof servers, comprising the steps of: providing at least one loadbalancer for routing requests to the plurality of servers; obtaining, atthe at least one load balancer, a request from a client; examining, atthe at least one load balancer, the request; estimating costs ofsatisfying the request by at least two of the plurality of servers basedon estimating at least one of a number and a cost of at least one remoteaccess for satisfying the request; and routing the request to a serverof the plurality of servers with a low estimated cost of satisfying therequest.
 2. The method of claim 1, wherein the step of routing isaccomplished by classifying a request into a partition and routing therequest to a server hosting the partition.
 3. The method of claim 1,wherein, in the step of providing at least one load balancer, the atleast one load balancer comprises at least one content-unaware loadbalancer routing requests to a plurality of content-aware loadbalancers.
 4. The method of claim 3, wherein, in the step of providingat least one load balancer, at least one of the plurality ofcontent-aware load balancers resides on at least one of the plurality ofservers.
 5. The method of claim 1, wherein the step of estimating costsfurther comprises the steps of: examining at least one parameterincluded in the request; and using information about how data arepartitioned among the plurality of servers to estimate at least one ofnumbers and costs of remote accesses for satisfying the request.
 6. Themethod of claim 1, wherein the step of obtaining a request furthercomprises obtaining a request that is sent using the TransmissionControl Protocol/Internet Protocol and wherein the step of examiningfurther comprises accepting a TCP connection.
 7. The method of claim 1,wherein the step of estimating costs comprises estimating at least oneof a measure of resource utilization and a desired service level forsatisfying the request.
 8. The method of claim 1, wherein the step ofestimating costs may vary with respect to at least one of a number ofservers and time.
 9. The method of claim 1, wherein, in the step ofobtaining, at the at least one load balancer, a request from a client,the request comprises a request for dynamic data.
 10. A method ofsatisfying requests in a system comprised of a plurality of servers,comprising the steps of: providing at least one load balancer forrouting requests to the plurality of servers; obtaining, at the at leastone load balancer, a request from a client; examining, at the at leastone load balancer, the request; estimating costs of satisfying therequest by at least two of the plurality of servers; sending, from theload balancer, information obtained in the estimating step to a serverof the plurality of servers along with the request; and using, at theserver, the information to satisfy the request.
 11. A system forsatisfying requests from at least one client, comprising: a plurality ofservers; at least one content-aware load balancer for routing requeststo the plurality of servers; and at least one cost analyzer associatedwith the at least one content-aware load balancer for estimating costsof satisfying requests by different servers based on at least one ofnumbers and costs of remote accesses for satisfying requests.
 12. Thesystem of claim 11, wherein the at least one content-aware load balancercomprises a plurality of content-aware load balancers and wherein thesystem further comprises at least one content-unaware load balancer forrouting requests to the plurality of content-aware load balancers. 13.The system of claim 12, wherein at least one of the plurality ofcontent-aware load balancers resides on at least one of the plurality ofservers.
 14. The system of claim 11, wherein a request is routed byclassifying the request into a partition and routing the request to aserver hosting the partition.
 15. The system of claim 11, wherein the atleast one cost analyzer estimates costs by examining at least oneparameter included in the request, and using information about how dataare partitioned among the plurality of servers to estimate at least oneof numbers and costs of remote accesses for satisfying the request. 16.The system of claim 11, wherein a request is sent using the TransmissionControl Protocol/Internet Protocol and a TCP connection is accepted whenthe request is examined.
 17. The system of claim 11, wherein the atleast one cost analyzer estimates at least one of a measure of resourceutilization and a desired service level for satisfying the request. 18.The system of claim 11, wherein estimating costs may vary with respectto at least one of a number of servers and time.
 19. Apparatus forsatisfying requests from at least one client in a system comprised of aplurality of servers, the apparatus comprising: a memory; and at leastone processor coupled to the memory and operative to: (i) obtain arequest from a client; (ii) examine content of the request; and (iii)estimate costs of satisfying the request by at least two of theplurality of servers based on estimating at least one of a number and acost of at least one remote access for satisfying the request.
 20. Amethod for satisfying requests from at least one client in a systemcomprised of a plurality of servers, comprising the steps of: obtaininga request from a client; examining content of the request; andestimating costs of satisfying the request by at least two of theplurality of servers based on estimating at least one of a number and acost of at least one remote access for satisfying the request.
 21. Anarticle of manufacture for use in satisfying requests from at least oneclient in a system comprised of a plurality of servers, comprising amachine readable medium containing one or more programs which whenexecuted implement the steps of: obtaining a request from a client;examining content of the request; and estimating costs of satisfying therequest by at least two of the plurality of servers based on estimatingat least one of a number and a cost of at least one remote access forsatisfying the request.
 22. A method for providing a load balancingservice, comprising the step of: a service provider providing a serviceto a customer which comprises: obtaining a request from a client;examining content of the request; and estimating costs of satisfying therequest by at least two of the plurality of servers based on estimatingat least one of a number and a cost of at least one remote access forsatisfying the request.